Ep 21: Replaying the MP3 of Life (Joe Parker)

Why do some rove beetles look like ants? Why do living things evolve similar solutions to common problems? Is there predictability within the evolutionary process?

On this episode, Art and Marty talk with Joe Parker, an entomologist at Caltech. Joe has been collecting beetles since the age of 16, when he first became amazed by their incredible diversity. He now focuses on rove beetles and studies their evolutionary relationship with ants to understand how different species converge upon similar traits.

Follow Joe on Twitter: @Pselaphinae

• Episode Transcript

  AW = Art Woods

  MM = Marty Martin

  JP = Joe Parker

  MM: Hey, this is Marty. I just wanted to give you a heads up that I'm headed off to Vietnam for a few weeks to do some research on my favorite bird, the house sparrow. That means I won't be around to record Big Biology, and we'll still release two episodes this month, but the second one will come out the last week of the month instead of the third one. We hope you enjoy this next episode with Joe Parker.

  AW: Some of the most successful predators in tropical forests are army ants. They send out legions of workers to seek out prey, and those workers totally overwhelm other insects, and sometimes even vertebrates like lizards, frogs, and birds.

  MM: When I was a graduate student working in Panama, we used to let them walk all over our feet and crawl up our legs so we could have a front row seat to see all the insects fleeing the swarm. But when I tried the same thing with old world army ants in Kenya, boy what a mistake! They're much more aggressive than neotropical ants.

  AW: While pretty much everything, including Marty, runs away from these things, there is a group of organisms that lives with them, and even takes advantage of them.

  MM: We're talking about rove beetles. They're a huge group of insects. There are roughly as many species of rove beetles as there are species of vertebrates combined, and a handful of these rove beetles have evolved to live with army ants.

  AW: These ant loving beetles look a lot like ants, even though other species of rove beetles have basically no resemblance, and some of them even steal pheromones from their hosts so that they smell like ants.

  MM: The con is so good that the ants let the beetles live in the nest, even though they contribute nothing to the colony. In fact, these beetles eat ant eggs and larvae, all without the ants noticing.

  JP: But those are really, really aggressive ant colonies. But you see how they interact with these beetles, and they treat them, you know, like royalty. They'll groom them, they'll clean them, they're absolutely accepting of these beetles into the nest. The beetles will groom the ants. In the case of the beetle grooming the ant, it's to steal the ant's colony pheromones, these cuticular hydrocarbons that they then smear over themselves to chemically disguise themselves. And they have to keep doing this, they have to keep intimately, you know, interacting physically with the ant to keep this sort of, uh, maintain the colony order and cloak themselves.

  AW: That's Joe Parker. He's a biologist at Cal Tech who's been collecting rove beetles since he was 16. Now he's studying them to understand a big mystery in biology -- why do living things keep converging on similar solutions to common problems?

  MM: This concept is called convergent evolution. It refers to examples where distantly related species come to evolve the same kinds of traits. For example, bats, birds, and butterflies have all evolved wings to fly.

  AW: Deserts in North America are dotted with cactuses, whereas deserts in Africa are home to a group of plants called Euphorbia that also have spines and fleshy tissues that store water. Cactuses and Euphorbs aren't closely related, but they converged on similar phenotypes for desert life.

  MM: The process of evolution is so contingent that the paleontologist Stephen J. Gould argued that its outcomes were basically unpredictable. If you could replay the MP3 of life, organisms in this reset world would look and act nothing like those we see today.

  AW: Rove beetles have evolved to look like ants at least a dozen times, which makes them a great set of species to investigate convergent evolution.

  MM: The rove beetle family tree makes it seem like maybe evolution is less contingent than Gould thought, and that's what Joe's lab is studying.

  AW: I'm Art Woods.

  MM: And I'm Marty Martin.

  AW: You're listening to Big Biology.

  [instrumental]

  03:32

  AW: I wanted to start by, um, kind of setting the stage, the big evolutionary stage, so, um, many of our listeners may know that Steven J. Gould had some, he wrote many books, many super interesting books, and in Wonderful Life in 1989, he made a big deal out of this idea of the contingency of evolution, and his thought experiment was, if you could go back in evolutionary time and sort of reset the tape, that it would play forward in a highly divergent way from what we actually have seen today. In other words, if you replay the tape of life, you get something different every time, and he really emphasized, sort of, contingency over things like repeatability and predictability of evolution. So, what do you think about Steven J. Gould's idea, and what's the kind of state of the field on, on that thinking?

  JP: Yeah. I think it's becoming clear that one of Gould's biggest legacies is this notion of contingency and, and really the kind of posing of this question of how predictable biological evolution is, and in his view, any replay of the tape of life would lead evolution down a very different path to the one that we see today, because in his opinion, evolution is inherently a kind of stochastic process in terms of, you know, mutation and environmental variation.

  AW: But, but why, so that seems like he's emphasizing, you know, the origin of variation on which selection can act, and deemphasizing the potential for there to be, you know, very strongly similar selection pressures across different lineages that might shape them to, you know, convergent outcomes, or the same outcome, sort of, regardless of the starting point. So, why do you think he emphasized the stochasticity over, you know, the potentially more predictable parts?

  JP: I mean, I think in what we know now is that evolution can be highly predictable for exactly those reasons, and so, I think, you know, we have all of these cases of convergent evolution where the same kinds of traits have arisen in response to similar selection pressures. So, you know, counter to Gould's view of contingency, there are all of these examples of, you know, many of them are extremely well studied that have shown that evolution can be a highly predictable process, so there are clades of organisms where similar traits have arisen multiple times over in response to similar selection pressures. So, there's clearly some element of kind of determinism or like bias in the evolutionary process. I would say that kind of, supporters of Gould would challenge these examples of convergent evolution and say that most of them are evolutionarily pretty young. So, we've got things like, you know, three spine sticklebacks, which show amazing repetitive evolution in terms of, you know, phenotypic traits like, you know, the skeletal morphology and these sorts of things.

  06:45

  AW: But those all arose over the last what, 10 or 15,000 years since the last [word?]

  JP: Exactly, you know, you're talking in some of those cases about the same mutation, which is, you know, same element of standing genetic variation that's been repeatedly selected when these lineages have transitioned from a marine to a freshwater environment. And so, it's to some extent unsurprising that in very young clades, at least a subset of lineages would evolve in the, in a similar kind of direction, because the starting pool, the genome is very similar, sometimes the, you know, the starting pool of selectable genetic variation is the same. I think supporters of Gould would argue that, you know, this is in the short term. If you were to come back in tens or hundreds of millions of years into the future, even these lineages would have started to diverge from each other. So, yeah. I think the real question is time, and this is something that I think has not been resolved. There are clearly examples of like, highly convergent traits evolving, sometimes with great frequency within certain clades. You think of these kind of radiations and island ecosystems, or lake ecosystems, where kind of, the similar, a similar natural experiment has happened in each case and you've got similar kind of, eco-morphs of fish, or morphed spider morphologies, or things like this evolving in these different kind of evolutionary replicas.

  AW: We want to come back and hit this idea of adaptive radiations pretty hard, maybe after we talk about some of the details of the natural history of your system, but Marty, did you want to jump in?

  MM: Yeah, so, I want to, before we go more into the convergent evolution or even adaptive radiation, Joe, do you know of literature that directly addresses this sort of scale at which the contingency is harder to swallow? So, you know, the finer levels of variation that we start to think about as, you know, for the reasons you just articulated is fine, but maybe it's at the levels of phyla that you sort of, the contingency framework makes a lot more sense, and as you narrow things down, you can end up with a sort of convergence happening more often for a lot of the reasons that you mentioned.

  JP: Sure, sure. So, and, so, yeah, there's kind of two answers to this question. So, first of all, there are clear examples of convergent traits arising in deep, you know, taxa which are widely separated, so the evolution of flight, you know, we saw in insects, [terrasols?], birds, bats, okay these are independent origins where there's really only one solution to that problem and you know, given the huge, you know, richness of animal life, they've evolved relatively infrequently, and there's always the same kind of solution to that.

  AW: Jet propulsion is not an option then.

  JP: Right, right. There's not really a kind of metazoan helicopter, right? Like, yeah, exactly. And so, you know, evolving to occupy like an, you know, aerial niche space you've, there's only kind of one solution to that problem, and sure enough, there are these four widely divergent lineages which have, you know, hit upon that same kind of solution. So, you know, fins are another example of this. Convergence on this level is kind of functional evolution of comparable...

  AW: There's one obvious solution.

  JP: Right. But, um, and so, clearly our examples of convergence at really deep, you know, deep divergences between lineages. I don't know if it helps to think really about phyla as kind of units of, because they're, to some extent, you know, taxonomic constructs. There was a really great paper by Terry Ord (?) a couple of years ago, I think it was a 2015 paper, where they scanned the literature for all of the examples they could find of convergent evolution and kind of rank them depending on how old the systems were that they were studying, and the drop off is after, you know, 10 or 20 million years, is like this, you know, as you go further past, you know, 10 or 20 million years, there's virtually kind of, no examples of really, really, you know, frequently evolving, highly convergent complex traits. And most of the really striking examples of convergence that are in that 10 to 20-million-year age bracket, actually really at the upper end of that, so if you know, tens of thousands to you know, a couple of million years old.

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  11:39

  AW: I wanted to ask your, just your thoughts on an event that I think of as a highly contingent one, and this is like, you know, almost kind of, you know, public knowledge contingent event, and that is the asteroid impact 66 million years ago that, you know, spelled the demise of the dinosaurs and I think the, you know, the public perception is that that cleared away a very dominant set of animals and made way for the rise of mammals.

  JP: The [word?] explosion.

  AW: Yeah. And, you know, there's no way to think about that except as a highly contingent event, because the asteroid might have missed the Earth if, you know...

  JP: Right, yeah, no, absolutely.

  AW: And you know, so, is that an example of the kind of Gouldian contingency over macroevolutionary time that he had in mind?

  JP: I would say that, yeah. I mean, that's environmental stochasticity like right...

  AW: [word?] large.

  JP: Yeah, the poster child of it. Yeah. I think, you know, and then it becomes a question of these further questions of contingency, you know, when, when there's a mass extinction event, and there's a kind of, all this unoccupied niche space, how does it get filled? Is that a kind of predictable process? Or, you know, if you were to replay the asteroid event many times, would you still get the emergence of birds and mammals, and, you know, ground group ants? That's the most important event that happened after the dinosaurs went extinct.

  MM: Of course! Hey, that's a good segue. So, you have done this amazing set of studies on a group of very strongly interacting organisms. So, rove beetles, and maybe you can tell us a little bit about rove beetles and their interactions with ants, and in particular army ants. So, can you just lay out the sort of basic natural history of that interaction?

  13:31

  JP: Right, so, um, I think everyone listening to this should know what rove beetles are. They are the largest family in the animal kingdom. There’re currently 64,000 described species of rove beetle. The family [phrase?], that's about the same size as the entire vertebrate sub-phylum, so you know, this, and you know, the diversity of rove beetles that remain to be found and described is probably tenfold the number that we know about, so this is a massive, massive chunk of the animal tree of life here. Um, and just to give you a kind of picture of what a rove beetle looks like, they don't look like typical beetles by and large, because they have short elytra. Now, elytra are the kind of key innovation, the beetles have these hardened wing cases that protect the flight wings. In rove beetles, the wing cases are short, and they leave the abdominal segments exposed, and the body is kind of elongate, and the abdominal segments are flexible and kind of telescope with each other, and this enables these rove beetles to move really rapidly to undulate through leaf litter and soil habitats. The majority of species chase down other species of arthropods, so they're predatory beetles. They are very small insects, usually they're under a centimeter most of the time. But they're fascinating for another reason, which is that from this kind of free-living, solitary, predatory, um, kind of, grand plan, they've repeatedly evolved into highly social organisms capable of infiltrating social insect colonies, in particular ants and termites. And when they do this...

  AW: And your word for that is they're myrmecophiles, right? Or myrmecoid. Quick etymological note on an entomological word -- myrmix is the Greek word for ant. So, a myrmecophile is something that loves, or in this case lives with ants. You'll hear us use that word a few more times.

  JP: Myrmecophiles, or termitophiles are the ones that live in termite colonies, and these lineages of transition to this kind of symbiotic lifestyle, referred to as a sort of social symbiosis, because they behave, really interact with it, kind of social host organisms, they really embody evolution in the extreme, where almost every dimension of their phenotypes evolves quite dramatically. So, there are changes in the behavior of these beetles, there are often changes in their morphology, so you know, sometimes they start to mimic their host organisms, there's changes in their chemistry, where they, you know, are able to synthesize and secrete behavior manipulating compounds that suppress antigressions, so the beetles can gain what we refer to as kind of social integration, acceptance inside ant colonies. So, it's a really dramatic kind of ecological and evolutionary transition, and it's happened over and over and over again in this one group of rove beetles that I study.

  AW: Can I ask a sort of logic question about how you sort of deduce the fact of multiple independent evolutions of these beetle lineages, and just to lay it out for the listener, you can imagine sort of two ways that you might get the diversity of beetles that you see that are myrmecophilous, right? And one is that there was a single evolutionary origin of that kind of lifestyle, and then there was a big radiation of rove beetles from that ancestral lineage. And the alternative is that it evolved multiple times. And just some, from a sort of macroevolutionary and comparative context, how do you distinguish those two ideas?

  17:33

  JP: So, what we did was collect these beetles from these horrible army ant colonies, and collect a lot of other rove beetle species, which did not live with ants, did not live with army ants, sequence several loci, so several different genes from these beetles, and reconstructed an evolutionary tree. And, you know, I wish I could, you know, the listeners could actually see this tree, because what you see is...

  AW: Yeah, all audio.

  JP: Yeah! There's this kind of forest of black, you know, black lineages, which are kind of, you can imagine, all of these lineages of just free living, kind of, generalist predatory beetles that have the typical rove beetle body plan. You kind of imagine this kind of backbone of the tree. We can color all of those lineages black, all right? And then sprinkled amongst this kind of forest of black branches, everything, there are all of these independent transitions where the branches have become, let's just say, orange for example. So, you see, all of these orange little branches popping up across the phylogeny, and these are all these independent evolutionary instances of these myrmecoid, these ant mimicking, army ant myrmecophiles, each of which has a similar morphology and similar set of behaviors. So, you can infer statistically that, you know, the ancestral condition was free living, you know, typical rove beetle morphology, and from this kind of ancestral starting condition, which is the majority of the 60,000 [or is it 16,000?] species of [word?], there have been at least 12, and maybe, you know, a couple of dozen independent evolutionary transitions to life inside ant colonies.

  AW: And just to be clear, the alternative, if they had all evolved from a single ancestral lineage, all of those...

  JP: They would have all clustered together.

  AW: All of those orange lineages would be clustered together instead of being dispersed across the tree?

  JP: With a single common ancestor, yeah. And so, historically, taxonomists had put most of these lineages into one tribe, so army ants are the subfamily Dorylinae, the [word?] with the Doryline ant mimics, and so, this kind of deity of Aleocharine taxonomy and systematics Charles Seevers, created this tribe that dumped all of these crazy ant mimics into it, and it's like...

  MM: Because they all share that trait, so almost all of each other's closest relatives, right?

  JP: Yeah, yeah, threw his hands up, he was like, well you know, I'm going to, the best I can do is kind of lump them together, because they're all so anatomically modified, it's hard to find traits that kind of unite them. He did his best to kind of do that, but it was obviously kind of not real, you know. They were completely unrelated to each other. And really, every different army ant genus has evolved, sort of picked up its own lineage, in some cases multiple lineages of these beetles that parasitize it, so really, nothing is, has not succumbed to these beetles and this kind of ecomorph that they repeatedly evolved.

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  20:35

  AW: So, just a couple natural history questions. First, so, to a casual observer, would you, if you saw a rove beetle, would you think it was an ant? I mean, do they look that similar?

  JP: I'm too far gone to see...

  AW: Well, you're not the casual observer! I'm talking about like...

  JP: I can't, I, I... So, I can only go on what I, the response I get when I show people a drawer of specimens, of rove beetle specimens. So, the first thing is oh,

  AW: It's oh I want to see the beetles, don't show me the ants!

  JP: Yeah, yeah! So, first of all, it's oh my goodness they're so tiny, how can you even see that, and secondly, oh they really do look like ants, don't they? And I, I kind of don't agree with that. So, when they, when they evolved to look like ants, they can really look like ants, but the ones that, you know, have not evolved that kind of morphological mimicry to me, are quite distinct. That said, I think compared to a lot of other beetles they're a little bit more ant-like. So, the body plan's probably fewer steps removed from ant-like than most other beetles, and I think this is again a kind of, maybe one of the pre-disposing factors that these beetles have which has enabled them to quite readily and convergently evolve ant-like morphology.

  MM: And remind us, how separated are the last beetle and ant ancestor?

  JP: So, this can evolve relatively rapidly, so just in a few million years. Basically, the stem lineage between free-living and like the ultimate manifestation of [word?] was 11 million years. We published a couple of years ago, actually 2014, so a few more years ago than that, a transitional fossil myrmecophile, which marks a kind of, I don't want to use missing link, but it almost is the perfect opportunity to use that phrase, because it really does embody...

  AW: Just go for it!

  JP: Yeah, let's go for it! It's, it is the tiktaalik of myrmecophiles.

  MM: There it is!

  JP: Yeah, yeah! It's got, you know, all, you know, five of its six legs in an ant colony. It, it, it embodies this kind of transitional phase between a free-living ancestor and like a highly socially integrated, obligate, kind of sociable symbiotic myrmecophile. And we could use this to kind of calibrate the phylogeny, the date of the phylogenetic tree of this particular radiation of these myrmecophiles.

  AW: I just wanted to ask here too, you talked a lot about the, you know, different ways that they're interacting. But what are the beetles getting from the ant colonies, and you know, I'm sure one of the answers is food, possibly protection...

  JP: Both, yeah, yeah, all of the above...

  AW: Can you just talk about...so, what are the resources they're getting?

  JP: Yeah, so, so, you, so, rove beetles are ancestrally predatory, right? They're these free-living generalist predators. And at the most fundamental level, all myrmecophiles are, are like extremely specialized predators which are targeting resources inside ant colonies. So, they feed on the brood, sometimes, so, the ant eggs and larvae, they sometimes...

  AW: So, they're slipping off and eating ant babies?

  JP: Exactly, yeah.

  AW: And the ants don't notice?

  JP: They don't, they don't notice. Sometimes they're like fully complicit in the whole thing. Sometimes they pick off...

  MM: Yeah, you had written that they, the ants pick up the beetles and move them to their broods, right?

  24:04

  JP: Oh absolutely, yes, yeah.

  MM: Oh, that's amazing.

  JP: In, in multiple cases, it's the kind of relationship so far gone that the beetles are so good at tricking the ant that the ants will pick the beetles up, carry them inside the nest, sometimes they'll deposit them right in the brood galleries where they'll just start munching on the eggs.

  MM: Wow, smorgasbord.

  JP: In some cases, the beetles are fed trophallactically mouth-to-mouth by the ants as if they were just nest mates or larvae or something like this.

  AW: And do they antennate the ants to get them to do that?

  JP: Yeah, yeah, so there's all this kind of, they've broken their like, [word?] code, and tricked them to do this. In some cases, the beetle will lay its eggs in the brood galleries and the larvae hatch out and secrete a chemical that instructs the worker ants to feed the beetles' larvae preferentially over their own larvae. So, it's like an amazing kind of brood parasitism that these, these beetles are able to, to carry out. So, yeah. They are, and they're absolutely free-loading social parasites that, you know, infiltrate ant colonies, contribute essentially nothing to the life of the colony.

  MM: Can I ask you a little bit on the sort of other side of the natural history -- how do the beetles not die? I can't imagine, and I talked on our last episode about my stupid encounter with army ants, my own personal encounter, I'm not going to recount that again. But I can't envision a worse kind of place to be than an army ant...

  JP: Yeah, I mean I collect these beetles, right? So, I know...

  MM: Well, that was my next question Joe, how do you get these beetles?

  25:41

  JP: Yeah, so...

  AW: He couldn't pick a crazier thing.

  JP: No, no, no, yeah, I mean, you just, it's, so first of all, it's totally worth it, right? Like...

  MM: Says you!

  AW: He's a true enthusiast.

  JP: No, no, like all of the stings and bites to find that tiny thing walking around, like the magical thing inside the net, it's just, you instantly forget about the pain.

  MM: Forget the anaphylaxis, I'm fine!

  JP: Yeah, no, exactly. You just won't feel anything once you, yeah. And so, yeah. They're really highly aggressive colonies of ants in some cases. Art mentioned that they have infiltrated army ant colonies, and this, we have published a paper on this in 2017. They've repeatedly evolved to live inside army ant colonies, and this kind of, is one of the most extreme manifestations of this socially parasitic phenomenon, because in these cases, they've evolved to look like the ants. And I don't think it's that they're kind of visually mimicking the ants to fool them, because the ants are basically blind, right? What we think is happening is that army ants use a lot of tactile nest mate recognition. They're constantly kind of, much more than other species of ants, always kind of, you know, antennating [?] each other, licking each other, and they do this to the beetles too, and I think this is what selects for the ant-like shape, so the beetle can pass tactile assessment inside these colonies. Often the beetle and the ant are a totally different color, but the beetle is ant enough like in form to integrate into the nest. But those are really, really aggressive ant colonies. But you see how they interact with these beetles, and they treat them, you know, like royalty. They're, they'll groom them, they'll clean them. They're absolutely accepting of these beetles into the nest. The beetles will groom the ants. In the case of the beetle grooming the ant, it's to steal the ant's colony pheromones, these cuticular hydrocarbons that they then smear over themselves to chemically disguise themselves. And they have to keep doing this, they have to keep intimately, you know, interacting physically with the ant to keep this sort of, maintain the colony odor and cloak themselves to make...

  AW: So, you've talked about this gland that the beetles have, and they use that to produce chemicals that subdue aggression by the ants, but, so, the beetles are not producing any of their own cuticular pheromones, they're getting all of those from the ants, is that right?

  JP: Or very little. So, this is one of the things we're studying in my lab. We, one of the species we work on is one of these grooming species, which actually walks on top of ants, it seems to sort of chemically subdue the ant, so the ant looks like it's almost gone to sleep. It'll walk on top of the ant, bite onto the ant's long first antennal segment, and that enables the beetle to kind of anchor itself there on top of the ant's body, and then it's got these specialized feet with these brushes on them, that it uses to scrape against the ant's body, and then it smears the pheromones over itself, and so you get this perfect match of cuticular hydrocarbons. If you take the beetle and the ant away from each other and like profile them using gas chromatography, mass spectrometry, you see peak for peak...

  AW: You can't tell them apart?

  JP: You basically can't tell them apart.

  AW: And so, the beetles have to do this all the time presumably, you know?

  JP: They, oh, they spend hours...

  AW: Cup of coffee in the morning and then go groom some ants?

  JP: Yeah, yeah, in time for another groom, yeah, exactly. Yeah. And so, one of the things that we've done is we've put synthetic cuticular hydrocarbons on the ant and you see them being transferred onto the beetle, and we've also looked at the carbon 13 isotopic signature of these particular cuticular hydrocarbons, and that tells you for definite if they're coming from the same source or not, and they appear to be. So, I don't think in this case the beetle's making any of its own cuticular hydrocarbons. It's stealing, we think, all of them from the ant. There are other cases where the, including another species that we study in lab that where the beetle synthesizes its own cuticular hydrocarbons. It's a kind of rough approximation for the kind of general profile that these ant colonies are producing. But the beetles can detect this, the ants can detect this beetle quite quickly, if you know, if they graze their antennae against it, maybe it passes for an ant. But if they actually antennate it, so they start tapping it, they recognize instantly it's not an ant. And what happens in those cases is the beetle flips its body around and like secretes this appeasement compound from what we think is the hind gut, the ants kind of gobble up and find really yummy, and don't attack the beetle, so the beetle kind of wanders off. So, this beetle is not as socially integrated, it's not grooming, it's not physically interacting, and this beetle lives more on the periphery of the nest, and so I think to gain access to the deeper parts of the colony, like the groomer beetles actually found in the brood chambers, and deep inside the nest. This one has to have the exact profile of the colony, and really the only way to do that is through physically stealing it from your hosts. So, many of these super specialized, like the ones that live with army ants, the one that we study in the lab that lives with velvety tree ants, they are physically interacting, grooming with the ants, grooming the ants to actually take, physically take the colony odor.

  31:05

  MM: There's too much cool stuff here, Joe. I'm sorry, I've got to ask you one more natural history kind of ecology question that weirdly never came in my brain and I don't remember reading, nor did we talk about earlier. How many individual beetles of the same species are in a nest, and do they battle? Do they fight each other?

  JP: This is uh, something that I'm really interested in, which is the population control of these beetles...

  MM: Exactly, yeah.

  JP: ...in cycle [word?]. And so, you have to think of the ant colony as a kind of super organism in many cases. You know, this is a collective of individuals that are kind of, you know, working together, and the infiltration of a colony by one of these beetles, species of these beetles, is you know, it's effectively a sort of social pathogen infecting that super organism. And I think the same sort of like, you know, epidemiological phenomena apply to these beetles, much the same that, the way that they would you know, unicellular pathogens and parasites infecting populations of individuals. And, you know, what seems to be the case is that as the beetles become kind of more specialized and socially integrated, their abundance goes down, and their pathogenicity I think, the you know, cumulative pathogenicity of the beetles inside the colonies also seems to be producing, because, just because there's this sort of selection from reduced parasite burden. What's, I think, mind blowing is the fact that one of the things we've noticed is that many of these highly integrated, socially integrated species like the species that we study in the lab that grooms the velvety tree ants, many of these ones of the myrmecoid ant mimics of army ant colonies, the females, you know, that, the abdomen of the female, the kind of gaster that mimics the ants, if you break it open there's just one huge egg inside there, right? And so, you know, it's this sort of transition from like, R to K selection I think that kind of goes hand in hand with this increasing specialization inside colonies. And so, I think the beetles start to invest in smaller numbers of really high-quality offspring, because they know that, you know, they've got what it takes to survive inside these colonies, and they do not want to undermine the...

  AW: Essentially infinite resources, right?

  JP: Right, right, yeah. And they don't want to undermine their hosts. They, lots of these lineages of beetles, they've, their wings have degenerated evolutionarily, so they're not very good at dispersing, but they're so wedded to their hosts that they have kind of, you know, I think there's been kind of selection to reduce the impact they have on colonies.

  MM: Wow.

  AW: Evolutionarily just are more aligned than they were at the beginning?

  JP: Right, yeah. And so, and, but, and so, this can also give kind of niche separation inside the, inside the nest between species which are more or less socially integrated. So, in colonies of velvety tree ants, we've got the groomer beetle like deep in the nest producing small numbers of really high-quality offspring. And at the periphery, and actually out, sometimes outside the nest, we've got the other beetle, which still has the, like the defensive gland that will, it will use against non-host ants. It's got this appeasement behavior where, you know, if it gets detected by ants it like, you know, squirts out some of this secretion that ants find really attractive. This is much less socially integrated. It reaches higher numbers, it, around colonies. And I think it's also kind of much more transferrable between nests because it makes this kind of approximate cuticular hydrocarbon profile, so this is more down the sort of R end of the spectrum of life's biostrategies. And so, these beetles are kind of spatially separated because of the different ecological niches inside the same ant colony.

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  35:14

  MM: Alright, Joe. I want to challenge you in a sense, on whether this phenotype is actually complex, and, I mean, you articulated many different ways, I think your answer is yes, based, I mean, you didn't explicitly say it, but I'll be surprised if you say no. The reason I'm challenging you is that somehow, the phenotypic complexity of a bullfrog lives in its molecular machinery, right? So, you can get a tadpole and a very, very different lifestyle and physiology and morphology behavior in that organism that when it matures, it becomes a profoundly different thing.

  JP: Right.

  MM: So, that is regulated by, you know, it's still a complex process by all means, but it's regulated by a relatively, I mean it's not straightforward and there's a lot of things that we don't know, but we do have some sense of how it happens. I guess my question is, do you know anything at the level of molecular processing, I mean the sort of developmental physiology that actually underpins the claim that this is complex? Or, is it equally possible that there are the same sort of modules that are induced in these organisms and it's just an issue of turning them on and the kind of the ancestrally, they might have you know, the ancestor of these organisms might have similar things? So, you mentioned poise for myrmecophily in this taxon generally, and I'm, you know, sort of want to hear about the molecular manifestations and such.

  JP: Right. So, you can distill down the evolutionary changes into, you know, several different processes, so one of them is behavioral, right? Primitively, these beetles, the free-living beetles, if they interact with an ant, they chemically defend themselves and run in the opposite direction, right? And so, the beetles have this kind of big chemical defense [word?] in their abdomen that they, we think, is a kind of preadaptation for being able to break and enter ant colonies, you know, keeping the ants at bay by deploying this chemical weapon, feeding on the brood and things inside the, inside the nests, enabling these beetles to fare much better than other insects would if they were to find themselves inside an ant colony, okay? And so, two things happened when these beetles make this transition to life inside ant societies. Their behavior undergoes this reversal where they're no longer fearful of ants, they're attracted to them, okay? And so, you have to envisage some change in neural architecture, kind of mediating this switch in polarity from you know, fear to attraction. And so, this involves a change in kind of innate behavior, your innate response of when you're faced with another species of organism, in this case an ant. And so, all of these different lineages have had to undergone neurobiologically this switch from like, defense and fear to actual attraction, and evolve all of these interesting kind of motor programs for grooming their ancestors didn't have but which ants do have, and so these beetles have evolved kind of ant-like behaviors more often that ants have, you know, each independent time that it's arisen. And so, you have to posit all of these behavioral changes, the most profound being this behavioral reversal. And so that's a quite complicated change to achieve neurobiologically.

  MM: Well, can I push back a little bit on that one too? I mean, I work on parasitism, and so, one of the hallmark examples of profound behavioral switch within a species has to do with infection with toxoplasmosis. You heard this story about when toxo gets into rodent brains, it changes the smell of what's an otherwise terribly aversive stimulus, cat urine, it turns it into something that's actually attractive, and the neurological pathway by which that happens is a fairly straightforward one. So, it is the sort of thing that could be co-opted over and over again by different lineages.

  39:07

  JP: Yeah, yeah, okay. So, I guess your counterargument is that like, behavioral reversal could be mediated by relatively rudimentary neurobiological change.

  MM: The same circuit sort of exploited over and over again, even if you get a profound change, because there's an organizational sort of role of that particular circuit for a complex syndrome [I think that's the word?] of behaviors.

  JP: Yeah, yeah. So, so, I would agree with you, and I would say that the initial switch from like, you know, you know, repressing your kind of innate defensive behavior, like the impulse to use this defensive gland, you know, it could be easily overridden by like, for example, an odor from the correct host ant species or something like this, you know. But, to then subsequently evolve all of these other additional behaviors where you're you know, evolving into a much more social organism, you're evolving new ways to interact with another species, often in the kind of language and on the terms of the other species of organism. Those I think, you could argue a not just kind of attractional repulsion. These are motor programs that you have to install in the brain. So, I think that's where the complexity comes in. I would say that one of the reasons that you get this kind of convergent switch towards, you know, attraction to ants in these beetles may be because that initial switch is kind of rudimentary, along the lines of what you mentioned for, you know, many other parasites. This is one of the things that we're very interested in in my lab, because you know, we have, we culture one of these beetles in the lab that just, still has its like, the classical defense gland, but it doesn't use it when it's interacting with its, with its actual host ant. If you take this beetle out and put it with any other you know, aggressive organism or other ant species, you're kind of back to square one, behaves like a free living beetle because it uses this like benzoquinones, these nasty compounds this beetle produces. It uses this defense gland against really everything except its true host ant. So, we think an odor or chemical, maybe cuticular hydrocarbon blend of the correct host ant species is suppressing this kind of innate urge to use this chemical defense, and instead the beetle is selecting, this is where the complicated stuff...it's selecting this other behavioral program where it's now producing this appeasement compound the ants find really attractive. So, not only has it got to suppress, you know, this like sort of really primitive defense response, it's got to evolve this other, you know, way of interacting, this other kind of motor program. I think that's where the complexity comes in.

  41:55

  MM: Wow, okay that's fascinating. Yeah. I don't remember reading that in the papers that we had looked at before, that's really cool

  JP: Yeah, and you know, and the other aspect of this is the chemical evolution, so you know, the beetle has the free-living, you know, backbone of this phylogeny, they produce these benzoquinone compounds from this abdominal glandular complex that's composed of two different cell types. If you imagine the sort of segmental plates, the tergites on the top of the beetle's abdomen, between segments 6 and 7 there's usually a bit of intersegmental membrane tissue there, right? But in these beetles, that tissue's invaginated to form a large kind of sac, okay, and the epidermal cells there differentiate as the secretory cell type, they make an alkane solvent that fill that gland with the undecane solvent. Behind those, behind that kind of reservoir, there are these additional glandular units that make the benzoquinones, these are those noxious, kind of aromatic compounds. But those benzoquinones are solid at room temperature. They're trafficked along ducts from those gland cells and dissolve in the alkane solvent to make this functional defensive secretion. So, there's this kind of synergism between these two different cell types that gives the beetles this kind of chemical weaponry to defend themselves against ants and run in the opposite...what happens in the symbiotic species is they basically reprogram the chemistry of those cells to make new kinds of compounds. So, in some species they're able to sort of repurpose that gland to make things like you know, alarm pheromone, which is much better at disorienting ants if they're inside colonies. What also often happens is that that gland degenerates because they don't need it anymore, they're socially integrated, they no longer need to make benzoquinones, but they evolve entirely new glandular systems and entirely new chemistries. We don't know for the most part what most of these compounds are, but they appear to exert really kind of profound behavior manipulating effects on ants. And so, you know, the breakdown and assembly of totally new secretory cell types is in each case, they're, you know, they're not homologous glandular cell types. They're totally new secretory structures capable of making new kinds of compounds, and so that I think is a you know, you have to invoke quite sophisticated genetic and developmental changes to evolve you know, new exocrine glands with new biosynthetic capabilities, and that is also happening convergently each time. And this is not to mention all of the morphological changes which can happen that can often be really dramatic, down to, you know, fine details of cuticle sculpturation that mimic the ant, so all of this stuff is all happening in parallel in these lineages. So, I would argue that each one of those is relatively complex, and in totality, it's a highly sophisticated set of changes which have been happening convergently.

  44:52

  MM: Yeah, okay, you've got me convinced. I'll accept the complexity there.

  AW: Check!

  MM: Check!

  AW: I wanted to just kind of step back here for our last question and ask about adaptive radiations more generally, so, I mean, I think you've convinced us that this is one of the more spectacular radiations of animals that's known in terms of you know, total diversity of species. You know, adaptive radiations are something that you hear a lot about around biology departments and in biology seminars, and you know, the idea is you get for some reason rapid diversification of some pre-existing lineage into many species...

  JP: Right.

  AW: ...that occupy many different niches. So, based on, you know, what you've discovered about ants, is there something more general to say about adaptive radiations altogether?

  JP: So, I...

  AW: Like, what primes lineages for that?

  JP: I don't know if what we're studying is really an adaptive radiation, right? Because you know, it...

  AW: You mean, it might be a radiation but not an adaptive radiation?

  JP: Well, there are adaptive changes, but it doesn't...many times that this evolves, it doesn't lead to an extensive amount of lineage clade degenesis, right? You know, the, for example, these army ant mimics, each time it evolves there are these small clades of like, a few species, each of which are associated with a single army ant species. Army ants themselves are not particularly diverse. You know, I think if army ants did, you know, each army ant genus that these beetles evolved to...if one of those blew up, these beetles would co-speciate with them. But they just, they haven't, and so, there are sort of these many, could you call them microradiations? Maybe that doesn't even make sense, but like, the um, what's happened I think, it's better to you know, adaptive radiation is the, you know, a burst of clade degenesis where all those lineages arise from a common ancestor. What you can really think about in this case is it's more the case that you know, ants as a kind of ecologically dominant taxon have presented a huge niche space for these beetles to kind of move into, and they haven't done it from a single origin and radiated. What they've done is it's convergently, convergent infiltration. And so, in totality, there's lots of lineages of these rove beetles, but they don't come from a single common ancestor.

  AW: I see. So, lots of kind of, parallel microradiations.

  JP: Right. I think that's a better way to think about it. There are examples where there appear to have been, you know, elements of like, elevated rates of clade degenesis in myrmecophile beetles. There's a group of carabid ground beetles, so this is outside of the staphylinidae, these are not rove beetles now, which are relatively species rich called the paussinae. There's like maybe, 600 something species of beetles. And in Madagascar, there's evidence that these paussine ground beetles have radiated really rapidly, so, you know, approaching the kind of levels that you see in cyclids and this kind of thing, in terms of like divergence per unit time, or whatever the measure is for you know, frequency of speciation events. And there, you know, I think the kind of model is that these beetles kind of got onto Madagascar and kind of radiated into all of these kind of unoccupied pheidole ant colonies. And so, I think if there's kind of open ant niche space, these beetles can invade it relatively quickly, and if there's a single evolutionary instance of it, maybe they can spread quite rapidly between, between host ant species to produce this effectively true adaptive radiation. I would say in these, in rove beetles, you know, it's been this kind of, these beetles were there from day one. You can go back in the fossil record to the earliest ants, 100 million years ago, and you can find evidence of like myrmecophiles and also termitophiles in Burmese amber, which would cause the earliest examples of you know, social ants and termites. So, I think as ants kind of like, radiated and rose to ecological dominance, you know, most of that's happened in the past 50 million years. These beetles have just always been there, kind of opportunisticly picking them off and convergently infiltrating them, so there's never been, it's more this sort of gradual, cruel, and convergent infiltration rather than actual adaptive radiation across ants.

  AW: Yeah, Joe. So, we often at the end of our podcasts ask our guests about just sort of gaze forward into the future and you tell us, what's your sort of biggest, craziest question and biggest, craziest approach to answering it that you can think of? You know, something that might play out over the next 5 or 10 years.

  JP: Well, I think, I can tell you what we're trying to do in my lab. So, we're using these beetles as a model to understand social and symbiotic evolution, molecular, cellular, and neurobiological levels, and so, what we're doing is, you know, transforming them into laboratory models with, you know, genetic and molecular tools. What we want to be able to do is understand what happens in their brains at the level of neural circuits when they make this transition from free-living to symbiotic, to really understand kind of, how social evolution happens in the, you know, in the brain. These beetles have been able to kind of make this transition so many times they're a great model to understand how this might happen. So, our approach is really to try and engineer these beetles to be able to, you know, visualize circuit activity when they're interacting with ants. This is both in the case of the free-living species and the symbiotic ones. We're also...

  AW: How are you going to look at the circuitry in action?

  51:04

  JP: So, we're trying to make transgenic beetles using transposons and CRISPR. My background, I took a kind of, not really a timeout from rove beetles, but I realized I needed to train in a real genetic model organism to be able to kind of realize the potential of rove beetles, so I spent over a decade as a Drosophila developmental geneticist. I trained in fly genetics and molecular biology. And that's really the high bar of insect, you know, genetic approaches, and what I'm trying to do now, we've identified species that we can culture in the lab which have an element of genetic tractability about them so we can harvest embryos for microinjecting them, we can, you know, use like tissue labeling approaches, build kind of genetic constructs, the same kinds of tools that people use in Drosophila, but we can now bring into these beetles to, you know, do things like, you know, knock in genetically encoded calcium sensors into their brain, tether them to walking balls, and image the brain to photo microscopy when we're introducing, you know, ant stimuli to them to see what's happening in the brains of free-living and symbiotic beetles. The other thing we're very interested in is this question of like, you know, synthesizing these different chemicals. These beetles are these sorts of chemical factories that produce all sorts of different compounds, you know, the free-living species are producing, you know, defensive compounds, very interested in how they evolve this capacity to do this. They, you know, they have these totally novel cell types that other insects don't have that are making things like, you know, alkanes and benzoquinones. And what we've been finding, we dissect out individual body segments, profile them with single cell RNA [word?] to look at how these new cell types are being assembled from, you know, much more ancient cell types in the beetle which are producing things like the cuticular hydrocarbon. So, there's kind of evolutionary co-option of enzymatic pathways that these beetles have reassembled into new pathways to make new chemistries. So, this kind of cell engineering problem, these beetles are kind of world champions at doing it. And so, understanding, you know, the chemical and behavioral basis for this sort of convergent evolutionary change on a molecular and cellular level, I think that's where you, you know, this is where the next level of studying convergence, it's not just, you know, looking at genomic sequence data. That's never going to be very satisfying. We want to actually understand how you get from the genome to the, you know, symbiotic phenotype so many times by studying actually the, you know, the brain and the glandular chemistry of these beetles, molecular and cellular level.

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  54:21

  MM: To carry out Gould's original thought experiment, we would need a time machine, or another Earth. We would replay the MP3 file, and compare how evolution played out in each place.

  AW: Without a time machine, we'll have to look for clues in the real world.

  MM: Thanks for listening to the episode today. If you like what you're hearing, please consider donating to the show through Patreon, and you can become a patron at www.patreon.com/bigbio.

  AW: Thanks to Matt Blois for scripting and producing the episode. Haley Hanson and Chloe Ramsay manage Big Biology's social media feeds, and thanks to Steve Lane for managing our website.

  MM: We also thank the University of South Florida, College of Public Health for financial support.

  AW: Music on this episode is from Poddington Bear and Blue Dot Sessions.

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